1. Field of the Invention
The present invention relates to micromechanical rotation rate sensors based on the Coriolis principle. More particularly, the invention pertains to a sensor, including two oscillators, in the form of plates arranged one above the other in layers in parallel planes, which may be excited to oscillate at right angles to such planes by an electrostatic drive.
2. Description of the Prior Art
A micromechanical rotation rate sensor of the above-described type is described in detail in the International patent application published under Serial number WO 96/38710. FIG. 6 (which corresponds to FIG. 9 of the International patent application) illustrates such device. As stated earlier, such a rotation rate sensor comprises two oscillators which are arranged one above the other, aligned in layers. An upper oscillator 60, seen in the perspective view of FIG. 6, and a corresponding lower oscillator arrangement in mirror-image symmetry that cannot be seen in the figure are hinged via a first spring 70 on an electrostatic drive 61 in the form of a plate that is connected via a second spring 69 to a plate-like support 62. The entire arrangement, connected as a row and composed of the oscillator 60, the drive 61 and the support 62, is held in a frame 68 by a cross spring joint 63, 63xe2x80x2. As can be seen from FIG. 6, each element 60, 61, 62, including associated frame 68, has two layers, i.e., it is produced from a composite wafer with the interposition of a thin insulation layer (not shown) composed, for example, of SiO2.
The upper two-layer frame 68 and the lower two-layer frame 68xe2x80x2 thereby enclose an entire oscillator structure produced from four wafer layers, allowing different potentials to be supplied via outer frame connections 64 to 67. FIG. 6 does not illustrate cover and base wafers provided with bushings for electrostatic (capacitive) excitation, signal read-out and reset (in a closed-loop system); such features are illustrated at FIG. 2 of the above-referenced International publication which is hereby incorporated by reference.
The particular advantage of the two-layer oscillator structure as illustrated in FIG. 6 is that no interference with the measured values results from reaction forces caused by oscillator movement, although comparatively large oscillation amplitudes can be achieved with small capacitor drive gaps in the region of the drive 61. Rotation rate read-out is also capacitive via surface electrodes (not shown) on the upper face of the support 62 and on the lower face of the mirror-image support 62xe2x80x2 (also not shown) with corresponding opposing electrodes on the cover or base wafer (not shown). The illustrated cross spring joint 63, 63xe2x80x2 is advantageous in that rotary movements caused by Coriolis forces, and thus capacitance changes, are readily transmitted while, in contrast, horizontal and vertical oscillations in this area are suppressed.
In the oscillator of FIG. 6, electrostatic excitation is considerably simplified, despite relatively large oscillation amplitudes, by the narrow drive gap in the region of the drive 61, and can be produced with comparatively low drive voltages (e.g., a few volts). DC isolation of excitation and read-out can also be readily achieved by skillful use of silicon dioxide layers. For very low rotation rates, however, capacitive coupling combined with electrical resistances (which can only be undershot with difficulty due to design constraints) of the springs 63, 69, 70 can result in the injection of interference signals that are considerably larger than the signal to be measured. Thus, identification and isolation of the rotation rate signal places stringent demands on the electrical complexity.
It is therefore the object of the present invention to reduce electrodynamic coupling between the electrostatic drive and the rotation rate read-out or the rotation rate output.
The preceding and other objects are addressed by the present invention which provides a micromechanical rotation rate sensor based on the Coriolis principle. Such sensor includes two plate-like oscillators arranged one above the other in two planes for excitation via an electrostatic drive to oscillate at right angles to the planes.
The oscillators are suspended in the direction of a rotation axis on opposite side edges via, in each case, at least one narrow spring web between an associated plate-like support lying in the same plane, through which signals are read out, and an associated drive plate element of the electrostatic drive lying in the same plane.
The two supports and the two drive plate elements are, in each case, spaced apart, one above the other, with a fixed plate element inserted therebetween in such a manner that a narrow drive gap to the two drive plate elements is defined in each case. The drive gap is considerably smaller than the distance between the oscillators.
The preceding and additional features of the invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout both the drawing figures and the written description.